Occasionally, the system will not be able to deliver any messages. In this state, messages will reside on the message queue while the MTA attempts to set aside the messages for a period of time (retry interval) until it reattempts the delivery. This will continue until the MTA gives up and returns the message to the sender. The reason a message is undeliverable is fairly unpredictable. A number of reasons such as network connectivity, busy destination server, network throttles, and so on, could explain why the message is undeliverable.
On a busy server, these temporarily stored messages can build up during periods of high volume activities. Such a build-up can potentially cause problems with disk space. To avoid these build-ups, tune the MTA to retry delivery at a faster rate.
The retry interval is set within the Channel Block configurations of the imta.cnf file. The structure of this file consists of two parts: rewrite rules and channel blocks. The channel blocks define the behavior of a particular disk queue and related processes. This discussion refers to the tcp_local channel. The tcp_local channel provides delivery to sites outside an enterprise's local network, in other words, to places over the Internet.
The retry interval setting of the tcp_local channel is initially set by the default channel block. The default channel block allows settings to be duplicated to avoid having repeated settings.
The following is the default channel block:
defaults notices 1 2 4 7 copywarnpost copysendpost postheadonly noswitchchannel immnonurgent maxjobs 7 defaulthost red.siroe.com red.siroe.com
First, the structure of the channel block consists of the channel name. In the example above, this is the default channel block, which will be applied to channels without these settings. The second part is a list of channel keywords.
The notices keyword specifies the amount of time that can elapse before message delivery notices (MDNs) are sent back to the sender. This keyword starts with the notices keyword followed by a set of numbers, which set the retry period. By default, the MTA will attempt delivery and send notices back to the sender. These notices come from “postmaster” to end-user inboxes.
In this example, the MTA will retry at a period of 1 day, 2 days, and 4 days. At 7 days, the MTA will return the message and regard the message as a failed delivery.
In many cases, the default setting of the MTA provides adequate performance. In some cases, you need to tune the MTA to avoid potential resource exhaustions, such as running out disk space for message queues. This is not a product limitation, but a limitation of the total Messaging Server system, which includes hardware and network resources.
In consideration of these possible disk size issues, deployments with a large number of users may not want to attempt message deliveries for much shorter intervals. If this is the case, study the documentation listed below.